'////; 



839 



is so constant that it ser\es as :i means by which cither of the sulci may !><> ideiitiiicil. 

 'I'lic marginal portion separates (lie parareiitral lobule from the prcciineiis (quadrate 

 lobule), and is wholly within the parietal lolie. One of the nioM constant twu' 

 the siilcus cinjruli i> that which marks oil' the paracentral lohule from the superior 

 frontal gyru.>. Another .-omctimo divides the paracentral lobule into its frontal and 

 parietal portions. The siilcus cinguli is developed from tuo and sometimes three 

 (anterior, middle, and poMerior) -eparate furrows, which later extend and fuse into 

 continuity. This method of its development may explain the irregularities fre- 

 quently met with and the fact that sometimes in the' adult the sulcus occurs in 

 separate pieces. 



The central sulcus (fissure of Rolando) is one of the principal landmarks of the 

 convex surface of the hemisphere. It separates the frontal from the parietal lobe, 



Fn;. {;_'!. MASAI, Si EtVAd <>i TIU: CKUIIIIKAI. HKMISIMIKHKS. (After Toldt, "Atlas of Human 

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and likewise divides the soma-sthet ic area of the pallium. Its upper end terminates 

 in and usuajl\- cuts the supero-mesial border of the hemisphere immediately in front 

 of the termination of the marginal portion of the sulcus ciniruli. Thence it pursues 

 an oblique thou^l sinuous course forwards across the convex surface of the hemi- 

 phere, forming on the aVBra^ an ansrle of about 7L' with the supero-mesial border 

 Rolandic angle), and terminates in the fronto-parietal operculum immediately 

 above the posterior ramus, and about 2-5 cm. behind the point of oriirin of the an- 

 terior ranu oj the lateral fissure. It rarely cuts through the fronto-parietal oper- 

 .ilum. In its sinuous course, two bends are most marked: (!) The superior genu 

 occurs at about the junction of the upper and middle thirds of the sulcus and is con- 

 cave forwards. It accommodates the greater part of that portion of the cortex 



